This invention relates to a nematic liquid crystal composition having positive dielectric anisotropy.
It is known that a nematic liquid crystal material (hereinafter referred to as "N-liquid crystal") can be used for display, light modulation and the like by utilizing its specific property that optical characteristics are changed under application of electric fields, magnetic fields, ultrasonic waves and the like. Such devices comprise, in general, an N-liquid crystal filled and supported between two substrates disposed to confront each other with a distance smaller than about 50 .mu., at least one of which is transparent, and the change in the molecular arrangement caused under application of electric fields, magnetic fields, ultrasonic waves and the like is utilized for light modulation. When electric fields are applied, electrode plates in which a thin conductive layer is applied on one surface of a support such as glass plates are used as said substrates.
Compounds forming such N-liquid crystal are divided into two types depending on the relationship between the molecular structure and dielectric property, one type being characterized in that the longer axis and electric dipole of N-liquid crystal moleculars are substantially vertical to each other (the N-liquid crystal of this type will hereinafter be referred to as "Nn-liquid crystal") and the other being characterized in that the longer axis and electric dipole of N-liquid crystal moleculars are substantially in parallel (the N-liquid crystal of this type will hereinafter be referred to as "Np-liquid crystal"). Accordingly, the Nn-liquid crystal indicates an N-liquid crystal having negative dielectric anisotropy, and the Np-liquid crystal indicates an N-liquid crystal having positive dielectric anisotropy.
The conventional Np-liquid crystal electro-optical devices are composed of a pair of electrode plates disposed to confront each other and a Np-liquid crystal layer interposed between said two plates. In this case, the molecular axes of the Np-liquid crystal are in parallel with the electrode face and are arranged in the substantially same direction in a plane parallel to the electrode plate. If seen from the direction vertical to the electrode plate, molecular axes are arranged in the state continuously distorted from one another between adjacent planes. Such an orientation of molecular axes is provided by rubbing the electrode face along one direction with cloths, paper or the like, piling the two electrodes thus treated so that the rubbing directions are at right angles to each other and injecting the Np-liquid crystal between so piled electrodes. The molecular axes near by the electrode face are oriented along the rubbing direction while the molecular axes within the layer of the liquid crystal are oriented in the state continuously distorted from one another. When polarized light passes through this liquid crystal layer, the polarization plane of the light is rotated depending on the degree of distortion. This distortion can be relaxed by application of an appropriate electric field. Accordingly, by adjusting the intensity of the electric field it is made possible to adjust rotation of the polarization plane of polarized light passing through the device.
When the Np-liquid crystal device is interposed between two polarizers, it changes from the light-shielding state to the light-transmitting state or from the light-tramsitting state to the light-shielding state depending on the applied voltage, and this light modulation can be utilized for display.
Since the light modulation process utilizing the Np-liquid crystal device provided between two polarizers disposed in such a way that the oscillation planes of light cross to each other exhibits a mechanism quite different from a Nn-liquid crystal electro-optical device, a higher contrast ratio can be obtained in use for display of a pattern. Further, the Np-liquid electro-optical device can be utilized for construction of a Boolean algebra generator, a logical product gate, a NOR-gate and a more complicated logical circuit. If Np-liquid crystal electro-optical devices capable of responding quickly are employed, it can be utilized for display of a three-dimensional television or moving picture.
The threshold voltage value causing DSM in Nn-liquid crystal electro-optical devices is about 7 to about 10 V.sub.RMS and the saturation voltages value is about 40 V.sub.RMS.sub.' and the Nn-liquid crystal electro-optical device is generally operated under about 25 to about 40 V.sub.RMS. In contrast, in the case of the Np-liquid crystal electro-optical device, the threshold voltage value is about 1.5 to 4 V.sub.RMS and the saturation voltage value is about 7 to about 10 V.sub.RMS. Accordingly, a lower voltage electric source can be used, and the consumption of electric power can be reduced and the life of the device can be prolonged. Further, in the case of the Np-liquid crystal electro-optical device, since the wavelength region of transmission light varies depending on the electric voltage within the range of the threshold voltage value to the saturation voltage value, it can be used for display of colors. Moreover, also an apparatus comprising the Np-liquid crystal device in which the direction of rubbing electrode faces are arranged in parallel can be used for display of colors.
In case an Np-liquid crystal electro-optical device is so constructed that the molecular axis directions are random between substrates, the light is scattered when no electric field is applied to the device, and when an electric field is applied, the quantity of transmission light increases because molecular axes are oriented vertically to the substrates. In this case, use of a polarizer is unnecessary.
Most of N-liquid crystals that are presently known in the art are Nn-liquid crystals, and N-liquid crystal materials and their compositions which take a nematic mesophase at room temperature and have a positive dielectric anisotropy are hardly known in the art.
In Japanese Patent Application KOKAI No. 18783/72, it is disclosed that a mixture of an Nn-liquid crystal material and 4-cyano-benzylidene-4'-n-alkylaniline is a liquid crystal composition having the same electro-optical characteristics as those of an Np-liquid crystal.